Low power and Lossy Networks (LLNs), e.g., sensor networks, have a myriad of applications, such as Smart Grid and Smart Cities. Various challenges are presented with LLNs, such as lossy links, low bandwidth, battery operation, low memory and/or processing capability, etc. In LLNs (such as low power wireless and/or power line communication or “PLC”), nodes may discover a wireless/PLC connectivity graph by observing metrics derived from communication with neighboring nodes. One critically important metric is a reachability rate, such as an expected transmission count (ETX), generally the inverse of packet success rate. An ETX of 1 is ideal as it represents no loss, while an ETX of 2 indicates that for every transmission, one retransmission is expected (on average).
ETX has been used and is being used in a number of networks, such as those operating according to a protocol called Routing Protocol for LLNs or “RPL”. RPL is a distance vector routing protocol that builds a Destination Oriented Directed Acyclic Graph (DODAG, or simply DAG) in addition to a set of features to bound the control traffic, support local (and slow) repair, etc. The RPL architecture provides a flexible method by which each node performs DODAG discovery, construction, and maintenance.
Knowledge of the ETX metric is most useful on the sending node to make informed routing decisions and appropriately compute the routing topology (e.g., the DAG built by RPL). On links that provide link-layer acknowledgements, computing ETX is trivial. The sender need only keep track of the number of packets sent and the number of acknowledgements received for those packets. Unfortunately, not all links provide link layer acknowledgment, in which case, the computation of the ETX becomes very challenging.